(i) Field of the Invention
This invention relates to a conference bridge.
(ii) Prior Art
Difficulties arise in communications networks, such as telephone networks, designed to permit multiple interconnections between a number of communications devices, such as where a number of telephones are to be interconnected for the purpose of permitting users to conduct a conference during which each speaker may speak and be heard as the occasion requires. In principle, without consideration of practical transmission performance limitations, if the telephones are analogue devices with two wire links therebetween, it is possible to interconnect a number of telephones for conference type calls provided the interconnection routes between the telephones are also two wire links. Where, however, four wire links are employed, such as where the signals are digitally encoded at some point in the communications circuit, an equivalent technique of direct multiple interconnection is not possible and arrangements must be made for switching the incoming and outgoing signals from each telephone in such a fashion as to direct the signals between telephones in a way which will permit intelligible communication. To separate incoming and outgoing signals from each two wire telephone link, a hybrid is used to convert two wire to four wire links. It is known to digitise the outgoing signals from each of the telephones and, by using a suitable switch, to block outgoing calls from all but one selected one of the telephones from reaching the remainder whilst directing the outgoing signal from the selected telephone to the remainder. Various means have been proposed to control such a switch. U.S. Pat. Nos. 3,958,084 and 3,947,639 describe an arrangement in which the determination of which telephone is to be selected for the time being as providing the outcoming signal to the remaining telephones is made in accordance with a comparison of the instantaneous magnitude of the signal from the last selected telephone with the magnitude of signals from the remaining telephones and operating to replace the prevailing selection by another in the event that certain conditions are fulfilled. More particularly, the conditions are if the instantaneous magnitude of a non-selected signal is greater than the corresponding instantaneous magnitude of the signal from the prevailing selection, and if that greater instantaneous magnitude is also greater than a predetermined threshold value. In the arrangement of these patents, an arrangement is provided for varying the threshold under certain conditions. More particularly, the threshold is determined as representing peak amplitudes of the signal from the selected telephone but, manipulation is made to decrease the threshold stepwise in the event that fresh samples of the selected signal fall below the preceding threshold. Thus, in the event that the selected signal output should fail to reach the threshold level for a succession of sampling intervals, the threshold will eventually reach a zero state unless supplanted by a threshold derived from a supplanting signal. This has the effect that it is relatively more difficult for a selected signal to be supplanted where other of the signals are of larger magnitude but are possible echoes in their local hybrids, whilst still permitting a relatively free transition to another selected signal in the case where the earlier selected signal has, say, fallen to zero for an acceptable interval.
A difficulty arises, however, in implementing devices as described above, called conference bridges, in that the signal levels from each of the telephones may not reach the bridge with equal signal levels for equal sound input to the respective telephones. This phenomena arises, for example, where there are differences in the links interconnecting the telephones with the bridge such as due to different line lengths or line characteristics. In this case, it will be seen that unless adjustments are made to the signal levels, improper operation may occur since a signal from a particular telephone which exhibits, naturally, a greater signal level than those from other telephones would tend to take precedence even where this was not desired.
An object of the present invention is to at least in part alleviate the above mentioned difficulty.